Magnetorheological (MR) devices for damping and controlling vibration and shock are known. MR devices may be of the "rotary-acting" or "linear-acting" variety, and can advantageously provide variable controlled torques or forces, as the case may be. Known MR devices include linear dampers, rotary brakes, and rotary clutches.
MR fluid devices, for example, typically include a housing or chamber that contains a quantity of magnetically controllable fluid, with a movable member, a piston or rotor, mounted for movement through the fluid in the housing. The housing and the movable member both include a magnetically permeable pole piece. A magnetic field generator (a coil or permanent magnet) produces a magnetic field across both pole pieces for directing the magnetic flux to desired regions of the controllable fluid.
MR fluid devices employ a Magnetorheological (MR) fluid comprised of soft-magnetic particles dispersed within a liquid carrier. Typical particles include carbonyl iron, and the like, having various shapes, but which are preferably spherical and have mean diameters of between about 0.1 .mu.m to about 500 .mu.m. The carrier fluids include low viscosity hydraulic oils, and the like. In operation, these MR fluids exhibit a thickening behavior (a rheology change) upon being exposed to a magnetic field. The higher the magnetic field strength in the fluid, the higher the damping/restraining force or torque that can be achieved within the MR device.
MR fluid devices are disclosed in U.S. patent application Ser. No. 08/304,005 entitled "Magnetorheological Fluid Devices And Process Of Controlling Force In Exercise Equipment Utilizing Same", U.S. patent application Ser. No. 08/613,704 entitled "Portable Controllable Fluid Rehabilitation Devices", U.S. application Ser. No. 08/674,371 entitled "Controllable Brake", U.S. patent application Ser. No. 08/674,179 entitled "Controllable Vibration Apparatus" and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention.
U.S. Pat. No. 3,962,595 to Eddens discloses a "Magnetic Particle Brake", commonly referred to as "Dry Particle Brakes" that includes a rotating member that is disposed in an annular space in a stationary member. A field-generating coil is mounted in an outer part of the stationary member. Magnetic particles are disposed in a space between the inner part of the stationary member and the rotating member. Other magnetic particle brakes are disclosed in U.S. Pat. No. 4,350,913 to Eddens and U.S. Pat. No. 4,575,103 to Pedu.
Exercise machines such as stationary bicycles, rowers, stair climbers, and ski machines, typically rely on some kind of resistance generating device to provide adjustable resistance to the exercise movements of the user. Conventional resistance systems used in exercise machines, for example, friction devices, have deficiencies in providing reliable control of the resistance setting and in duration over repeated use of the machine. Before the present invention, however, there has been no resistance device for exercise machines using MR technology to replace conventional resistance devices and provide improved performance, reliability and endurance characteristics.
The foregoing illustrates limitations known to existing present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.